The adaption of a conventional altimeter instrument for use in indicating a theoretical glidepath in connection with the landing of aircraft is an art recognized concept. For example, U.S. Pat. No. 2,732,538 to De Celles discloses mechanism cooperable with the altimeter and distance measuring equipment of an aircraft to produce glidepath altitude-reference indications comparable by the pilot of the aircraft with his observation of altitude indications of the altimeter. This prior art device for providing an indication of a glidepath was particularly complex in the mechanism which it employed and was therefore quite expensive to manufacture. Of course, many aircraft are not provided with distance measuring equipment in which case the prior art glidepath indicator could not be employed. In any event, prior art glidepath indicator devices of this type were undesirable from the standpoint that readings provided by the distance measurng equipment of an aircraft are relatively accurate when the aircraft is distant from the landing area but are subject to error when the aircraft is in proximity to the landing area, as during a landing approach thereto. Moreover, even though some aircraft are provided with distance measuring equipment, many airports are not provided with the necessary ground instrumentation to permit use of distance measuring equipment for purposes of indicating a glidepath during a landing approach to such airports. In fact, a substantial number of smaller airports include minimal ground instrumentation which only include means for providing a final approach fix which allows the pilot to align the course of the aircraft with the direction of a runway in the landing area.
In connection with these latter mentioned airports, it can be appreciated that prior art glidepath indicating devices of the type used in conjunction with distance measuring equipment are completely incapable of providing an indication of a glidepath to a landing area in such airports, during an approach to the latter under instrument flight rules. This creates a potentially hazardous situation since pilots flying into airports provided with minimal ground instrumentation are sometimes forced to employ rather unsafe techniques for approaching the landing area including either waiting until the aircraft is in close proximity to the landing area and then diving steeply to the minimum descent altitude (MDA) associated with the landing area, thereby risking overshooting the landing area, or by significantly reducing the aircraft's altitude to the MDA well in advance of the landing area, thereby risking collision with tall obstacles laying in the aircraft's path of travel. Thus, there is a clear need in the art for a system which provides an indication of a safe, theoretical glidepath to be followed by an aircraft in making landing approaches to landing areas provided with minimal ground instrumentation.
Another type of prior art glidepath indicator is disclosed in U.S. Pat. No. 3,505,504 wherein an electromechanical system is employed for providing an indication of a theoretical glideslope path using a mechanical pointer driven by an electrical motor. This type of device was not successful in gaining widespread use because of its relative complexity and concomitant expense, in addition to the fact that the device lacked the high reliability demanded by the circumstances under which it was used; error in indicator readings were common because of fluctuation in the speed of the motor used to drive the mechanical indicator.